Screwdrivers are widely used to assemble screws onto a workpiece or dismantle screws. The existing screwdriver is a hand-operated screwdriver or an electric screwdriver. When the hand-operated screwdriver is used to assemble or dismantle screws, a user rotates the hand-operated screwdriver hardly, and sometimes needs to use two hands to handle the hand-operated screwdriver. An electric screwdriver can assemble or dismantle screws automatically, but the electric screwdriver is heavy and large, so the electric screwdriver is not easy to carry. Thus an automatic screwdriver with light weight and small volume is needed.